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Minecraft uses the Client-server model, that is users install the game client and connect to a server to play the game. Fabric allows mods to target either the Minecraft client or the Minecraft server, but also both at the same time.

The concept of client/server in Minecraft is ambiguous and may refer to either physical or logical sides. The terms client/server may be used to distinguish the different distributions of Minecraft (the Minecraft client vs. a dedicated Minecraft server) which are called “physical” sides. However, a Minecraft client hosts its own integrated server for singleplayer and LAN sessions, which means that a Minecraft client also contains server logic. Therefore, client/server may also be used to distinguish parts of the game logic which are called “logical” sides.

For both types of sides, there is a 'client' and a 'server'. However, a logical client is not equivalent to a physical client, and a logical server is not equivalent to a physical server either. A logical client is instead hosted by a physical client, and a logical server is hosted by either a physical server or a physical client.

The logical sides are central in the architecture of both distributions of Minecraft. Therefore, an understanding of logical sides is vital for any mod development with Fabric.

Physical Sides

The physical sides or the environment refer to the two distributions (jars) of Minecraft, the client (what the vanilla launcher launches), and the server (download available on for free). A physical side refers to which code is available in the current environment.

The client and server environment are minified distributions of the same program, containing only the required parts of the code.

In Fabric, you can often see annotations like @Environment(EnvType.CLIENT). This indicates that some code is present only in one environment; in this example, the client.

In Fabric fabric.mod.json and the mixin config, the client/server refers to the environment.

Each physical side ships classes used by its entry point and the data generator classes with entrypoint

Logical Sides

The logical sides are responsible for the actual game logic. The logical client does rendering, sends player inputs to the server, handles resource packs, and partially simulates the game world. The server handles the core game logic, data packs, and maintains the true state of the game world.

The client maintains a partial replica of the server's world, with copies of objects such as:
These replicated objects allow clients and servers to perform some common game logic. The client can interact with these objects while the server is responsible for keeping them in sync. Usually, to distinguish objects on the logical clients from the ones on the logical server you would access the world of the object and check its isClient field. This can be used to perform authoritative actions on the server such as spawning entities, and to simulate actions on the client. This technique is necessary to avoid desynchronization between the two logical sides.

Detailed look into all sides

With an understanding of which sides there are and how to distinguish between them, we can now look at each side in detail.

Physical Client

The physical client is the minecraft jar downloaded by the vanilla launcher. It contains a logical client and a logical server (integrated server). Its entrypoint is net.minecraft.client.main.Main.

A physical client can load several different worlds, each within a separate logical server, but only one at a time.

Compared to the logical server of the physical server (dedicated server), the logical server of the physical client (integrated server) can be controlled by the logical client on the physical client (e.g., F3+T reloads data packs and shutting down the client also shuts down the integrated server). It can also load resource packs bundled in a world to the logical client on the physical client.

All the logical client content is exclusive to the physical client, therefore you see many environment annotations on rendering, sound, and other logical client code.

Some mods target physical clients exclusively, for instance, Liteloader, Optifine, and Minecraft PvP clients (Badlion, Hyperium).

Physical Server

The physical server is the java dedicated server. Compared to a physical client, it only has a logical server (dedicated server). Its entrypoint is net.minecraft.server.MinecraftServer and the physical server can only have one world during its runtime. If a server should switch to another world, a server restart is required.

Its logical server differs slightly from that of a physical client as only one logical server instance is ever present when the physical server is running. Moreover, the logical server of the physical server can be controlled remotely via Rcon, has a config file called, and can send server resource packs.

Despite these differences, most mods are applicable without problems to the logical servers of both the physical client and the physical server as long as they do not refer to logical client content.

Its features of single world and resource pack sending, however, make vanilla mod (data pack and resource pack combination) installation much easier compared to on clients, as vanilla physical clients set up when connecting to the server automatically.

Some mods target physical server exclusively. For instance, Bukkit and its derivatives (Spigot, Paper, Cauldron, Xxx-Bukkit hybrids) always run on the physical server.

Logical Client

The logical client is the interface to the player. Rendering (LWJGL), resource pack, player input handling, and sounds happen on the logical client. It is not present on the physical server.

Logical Server

The logical server is where most of the game logic is executed. Data packs, world updates, block entity and entity ticks, mob AI, game/world-saving, and world generation, happen on the logical server.

The logical server on the physical client is called the “Integrated Server”, while the logical server on the physical server is called the “Dedicated Server” (which is also the name of the physical server itself).

The logical server runs in its own main thread, even on physical servers, and has a few worker threads. The lifetime of a logical server depends on the physical side it is hosted on. On a physical server, a logical server exists for as long as the process is running. On a physical client, multiple logical servers may be created, but only one logical server may exist at a time. A new logical server is created when the player loads a local save and closed when the player closes the local save.

Most universal mods target the logical server so that they can work both in single player and multi player scenarios.


The only correct way to exchange data between logical clients and servers is by exchanging packets. The packets (as documented on are sent between logical clients and logical servers, not physical sides. Mods can add packets to transfer custom information between two logical sides. Packets are exchanged in-memory for a logical client connected to its own integrated server, and exchanged over a networking protocol otherwise.

Logical clients send C2S (Client-To-Server) packets to the logical server. The logical server sends S2C (Server-To-Client) packets the logical clients. Packets are sent by a write method in a network thread and received by a call to a read method in a network thread.

For more details on how to handle networking, see this article.

Common misconceptions about logical servers

Most of the time, mods exclusively targeting the physical server also work on logical servers inside of physical clients.

However, modders for physical servers usually have assumptions which do not apply to integrated servers, including but not limited to:

  • Only one logical server instance exists on one game run
  • The world and entities should always calculate the game logic (i.e., the isClient field of the world object is always false)
  • Remote control, resource pack sending, and Favicon are present

These assumptions need to be corrected to make mods that run on logical servers.


Possible combinations of physical and logical sides:

Logical Client Logical Server
Physical Client Singleton Always Exists Exists when in local save; new instance for each play
Physical Server Does Not Exist Singleton Always Exists

Ultimately, the main confusion comes from the fact that logical servers exist on physical clients.

tutorial/side.txt · Last modified: 2022/04/13 04:32 by